The soldier said that in frustration at the standard-issue headgear, he had bought his own, American-made helmet because it fitted better and can be worn while shooting.

Requests for such helmets to be issued by the Army itself have been unsuccessful, the sniper wrote.

“Why is it near-impossible for us to be issued with potentially life-saving equipment required for us to carry out our jobs properly and safely?”

Responding to the letter, Maj Alex Mills of the Army’s Capability Directorate, suggested that snipers are wearing the wrong size of helmet.

He said: “The problem relates to the form, fit and function of the headwear. Basically, the difficulty can be largely solved by ensuring a correct fit of the Mk 7.”

Equipment specialists will now work with snipers preparing to deploy to Afghanistan to ensure their helmets fit properly.

The Mk 7 helmet has been in service since 2009, but is likely to be replaced from 2015 when the Army introduces a new “personal protective equipment” system which promises integrated protection for the head, shoulders and torso.

The Ministry of Defence said that the current helmet was the best available, offering better protection than commercially-available alternatives.

The MoD said: “The Mk 7 helmet gives our troops the best protection available. Trials show that other products are not as effective against blasts or blunt impacts. Training ensures snipers correctly fit their helmets to integrate with their body armour which reduces the risk of any impact on their ability to fire.”